Display devices are widely used such as liquid crystal display devices and organic EL display devices. In active matrix type display devices among such display devices, a thin-film semiconductor device called a thin-film transistor (TFT) is used. A TFT includes a gate electrode, a source electrode, a drain electrode, a semiconductor layer, a gate insulating film, and so on. The semiconductor layer included in the TFT is composed of amorphous silicon or the like is used. The semiconductor layer has a channel region where moving of carriers is controlled by voltage of the gate electrode.
In recent years, because of increase in definition and frame rate of screens in active matrix type display devices, there is a demand for increase in performance of a TFT such that the TFT is electrically connected with an organic EL element in such active matrix type display devices. Against this background, research and development have been actively conducted on TFTs using, as a semiconductor layer of a TFT, oxide semiconductor such as zinc oxide (ZnO), indium gallium oxide (InGaO), and indium gallium zinc oxide (InGaZnO, IGZO) (for example, Patent Literature 1). According to a TFT using oxide semiconductor, it is possible to realize high electrical properties according to which carrier mobility is high and off-current is reduced, compared with a TFT using conventional amorphous silicon. On the other hand, in order to improve carrier mobility in silicon semiconductor, crystallization needs to be performed by heat anneal, laser anneal, or the like. However, oxide semiconductor such as IGZO even having the amorphous structure is expected to have high carrier mobility, and accordingly does not need to be crystallized. Therefore, development has been proactively proceeded on a TFT using oxide semiconductor having the amorphous structure because such a TFT can be formed through process at a low temperature around a room temperature.
Unfortunately, there has been reported that intrusion of hydrogen into an oxide semiconductor layer deteriorates film properties of the oxide semiconductor layer (Non-Patent Literature 1). Here, deterioration in film properties of the oxide semiconductor layer means that resistance of the oxide semiconductor layer decreases. This varies threshold voltage of the TFT using oxide semiconductor, and as a result the oxide semiconductor layer is made to be conductive. Furthermore, reliability decreases. Here, decrease in reliability means that the TFT cannot operate as a transistor and the TFT cannot be stably driven as a transistor in a medium and long term due to shift in threshold voltage. Note that hydrogen which intrudes into the oxide semiconductor layer is considered to be in the form of gas, atom, radical, or ion.
In response to this, as a hydrogen protection film for protecting an oxide semiconductor layer against hydrogen, there has been proposed a method of forming a hydrogen protection film composed of aluminum oxide (AlOx) or the like (Patent Literature 2).